Investigators are trying to track down at least three men believed to be involved in a "vehicle-borne" terrorist attack anticipated in NYC or Washington D.C. to coincide with the tenth anniversary of 9/11. The three men traveled by plane to the U.S. last month from Afghanistan, one law enforcement official tells the Times, adding that one of the men is an American citizen. ABC reports that two of the individuals may have had U.S. documentation, and hears they came from tribal areas of Pakistan, where the threat intelligence originated.

Authorities have not yet identified the men, but they're currently searching multiple names of individuals who entered the U.S. after mid-August. Appearing on Good Morning America this morning, Vice President Biden described the threat as credible, but added that "there’s no certitude... We don’t have the smoking gun, but we do have talk about using a car bomb." An unidentified intelligence official tells ABC the information on the plot was "very specific... It seems like the information has been worked for a while." It's believed the alleged attack has been launched by new al Qaeda chief Ayman al-Zawahiri, Osama bin Laden's successor.

"We know from Bin Laden's own handwriting he wanted to do an attack around the anniversary," says Richard Clarke, former White House counterterrorism advisor and now an ABC News consultant. "We know from his successor's own audio tapes and video tapes that he feels he has to prove al Qaeda is still alive by avenging bin Laden's death." Speaking on his weekly radio appearance this morning, Mayor Bloomberg explained, "Credible means it's possible to do. If someone says they're going to do something so outlandish, they're never going to get the resources. They couldn't possibly do it. But something that is possible."

In NYC, traffic on several bridges and tunnels was snarled this morning by police checkpoints, which were also set up in Times Square and 34th Street. "NYPD is stopping and looking in EVERY vehicle at that [34th and 6th Ave," writes area man Chris Nichols on Twitter. "Have never seen that, and I've lived here 15 yrs." Bag checks at subway stations have also been stepped up, and NYPD officers have had an extra four hours added to their shifts, effectively increasing the number of cops on duty by a third.